In recent years, an image display device used by being mounted on a person's head has been developed. As such an image display device, there is a non-transmission type which covers the eyes and a transmission type which does not cover the eyes. In the non-transmission type, only an image is displayed on a display unit. Therefore, a user can view only the image. In the transmission type, a display unit, for example, is a half mirror. Therefore, a user can view both the outside environment and the image.
Since such an image display device is mounted on the head of a user, when the user moves his/her head, there may be cases where the outside or the image is hard to see. In contrast, an image display device capable of reliably transmitting character information serving as an image even when a user moves his/her head has been proposed (for example, see Patent Literature 1).